Ask OG: Rain Barrel Hygiene

Should I drain or clean out the water in my rain barrel that drains from the asphalt roof?

June 30, 2011

Q. While draining the last bit of water out of my rain barrel, I noticed that the water at the bottom of the barrel was almost black and full of sediment that appeared to come from the asphalt shingles on the roof. Are there hazardous chemicals in asphalt shingles? Is there a potential problem with using the water that drains off of them? Should I periodically drain out all the water to clean out the sediment in the bottom of the rain barrel?

Tom Wirth
Falls Church, Virginia


A. Like the water in the bottom of your rain barrel, this issue is murky and unlikely to clear up anytime soon. In addition to any compounds that may come from the shingles themselves—these may include hydrocarbons, according to some studies—the water from your roof may also contain pollutants from local industry or agriculture, pollens, molds, fungi, and droppings from birds or other wildlife. The expense of testing roof-collected rainwater for hazardous compounds means that very little testing has been done. A further complication is that water quality is site- or region-specific, explains Lenny Librizzi, assistant director, Open Space Greening, GrowNYC (formerly Council on the Environment of NYC). “You can’t really generalize,” he says. You can’t compare an asphalt roof in an urban area with a rural one where birds regularly roost. “The [varying] amount of rainfall and frequency means that if you tested the water in your barrel each time it rained, you would get a different result.” Since testing is prohibitively expensive, Librizzi adds, “I tell folks if they are uncomfortable with watering their edibles with collected rainwater, they should use it only for ornamentals. They will still be saving water.”

Using or not using water from your rain barrel on edible crops is largely a personal decision. Many gardeners consider this to be a safe and environmentally sound practice. If you do use water collected from your downspouts on vegetables and herbs, there are a few things you can do to minimize potential health hazards:

  • Consider adding a “first flush” feature that captures the first 5 to 10 gallons of water that comes off of your roof and holds it separately from subsequent water that goes into the main storage tank. This “first flush” contains the majority of the dust, pollution, bird waste, etc., that builds up between rains. You can use this water on ornamentals or lawn away from your vegetable gardens.
  • Apply water from your rain barrel to the soil around your plants rather than on the plants themselves. This allows the soil to perform its role as a filter and, if you amend your soil with plenty of compost, helps to trap compounds like heavy metals so they are not taken up by your plants.
  • Rinse produce from your gardens thoroughly with clean, potable water before you eat it.
  • Keep your rain barrel reasonably clean. Rinse it thoroughly at the end of each growing season and as you have the opportunity throughout the summer. If you notice that its contents seem particularly mucky or smelly, drain the barrel, rinse it out, and start afresh with the next rainfall.